(1) Field of the Invention
The present invention relates to a rubber composition which has large hysteresis loss, excellent fracture strength and good processability.
(2) Description of the Prior Art
As the rubber materials having large hysteresis loss, e.g., for tires, rubber vibration insulators and industrial products, there have been heretofore used butyl rubber, halogenated butyl rubber, ethylene-propylene diene rubber, emulsion-polymerized styrene-butadiene copolymer rubber containing a high content of styrene, polybutadiene rubber containing a high content of 1,2-double bonds, and polyisoprene rubber containing a high content of 3,4-double bonds and so on.
However, these rubber materials having large hysteresis loss are inferior in the covulcanizable property with diene rubbers such as high cis-polyisoprene rubber, natural rubber, high cis-polybutadiene rubber, emulsion polymerized styrene-butadiene rubber and the like, and have poorer fracture strength. Further, the smaller the dependency upon temperature of the hysteresis loss, that is, the value of loss tangent (tan .delta.) in a range of 25.degree.-70.degree. C., the more suitable is the rubber composition for the tires and the rubber insulation vibrators. However, it can not be said that the above-recited rubber materials possess satisfactory performances since they have large dependency upon temperature of the loss tangent.
Japanese Patent Application Laid-open Nos. 70,134/1982, 70,136/1982 and 70,137/1982 describe that the processability, the wet-skid performance, and the wear resistance are improved by using SBR containing a high content of vinyl bonds with a high content of styrene. But, such an SBR can not maintain high hysteresis loss characteristics over a wide range.
There is described, for instance, Japanese Patent Application Laid-open Nos. 180,615/1982, 180,646/1982, 4,633/1984 and 4,644/1984 that the wet-skid performance, rolling friction characteristics, and processability are improved by blending polymers having different molecular weights to obtain polymodal distribution of the molecular weight, but it is an actual situation that these diene rubber compositions do not afford high hysteresis loss characteristics or excellent balance between the hysteresis loss characteristics and the fracture characteristics.